1. Field of the Invention
The present invention relates to an optical scanning devices and an image forming apparatus.
2. Description of the Related Art
As an example of an image forming apparatus that forms an image by using the Carlson process, an image forming apparatus has been known in which, a surface of a rotating photosensitive drum is scanned with light beams to form a latent image thereon and a toner image developed from the latent image is fixed onto a sheet as a recording medium to form an image. In recent years, image forming apparatuses of this type have been often used for quick printing as an on-demand printing system, and there have been ever increasing demands for high-image density and high-speed image output.
In general, high-speed image output can be achieved by increasing the number of revolutions of a deflector that deflects light beams and the number of revolutions of the photosensitive drum to increase the printing speed. However, if the number of revolutions of the deflector increases, vibrations and noise occurring from its driving system increase and, power consumption also increases, resulting in low durability of the apparatus. Besides, because high-image density and high-speed image output have a trade-off relation, if the number of revolutions of the deflector increases, image quality is impaired.
As one method of achieving both high-image density and high-speed image output, an image forming apparatus has been proposed in which a light source is formed as a multi-beam unit and a photosensitive drum is scanned with a plurality of light beams at one time. In this image forming apparatus, diverging rays from a vertical cavity surface-emitting laser (VCSEL) serving as a light source of light beams and having a plurality of light-emitting sources are collectively deflected by a deflector. Thus, the photosensitive drum can be scanned with the light beams simultaneously.
For example, Japanese Patent Application Laid-open Publication No. 2003-270577 discloses an optical scanning device using an overfield optical system has been suggested in which the width of the deflecting surface of the deflector that deflects a plurality of light beams is smaller than the radiation range of the light beams. By using such an overfield optical system, the width of the deflecting surface can be decreased. Therefore, compared with a conventional deflector with the same diameter (a diameter of a circle in which the deflector is inscribed with a rotational axis as a center), more deflecting surfaces can be provided. For this reason, the scanning speed can be increased without increasing the number of revolutions of the deflector.
However, in the optical scanning device using an overfield optical system, the distribution of the light amount of the light beams reflected upon the deflecting surface is disadvantageously uneven. Moreover, in the VCSEL explained above, for example, because the angle of light-beam divergence is small, the distribution of the light amount the light beams incident upon the deflecting surface is slightly uneven compared with an edge-emitting laser and the like. Therefore, to combine the surface-emitting light source and the overfield optical system together, a technology is required that equalizes the distribution of the light amount of the light beams.